Electrical safety system for providing overcurrent protection of an electrical circuit in a vehicle

ABSTRACT

An electrical safety system comprises a main safety device including a N-type transistor and an auxiliary safety device including a P-type transistor, alternately activated under command of a controller. The N-type transistor and the P-type transistor have the function of overcurrent protection, respectively in a first operating mode and in a second operating mode. The auxiliary safety device includes a passive component, connected in series with the P-type transistor, for providing a voltage drop when a current passes through the passive component, and a driving circuit for turning off the P-type transistor under control of the voltage drop exceeding a first threshold, in the second operating mode.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/997,039, filed on Aug. 19, 2020, which claims priority to EuropeanPatent Application No. EP 19194381.0, filed on 29 Aug. 2019.

TECHNICAL FIELD

The present disclosure relates to the field of electrical safety systemsor devices for providing overcurrent protection of an electricalcircuit. Such a system can be used for example in a vehicle to protectthe devices of an electrical circuit of the vehicle, in case ofmalfunction.

BACKGROUND

Conventionally, vehicles use electrical fuses including a metal wire orstrip that melts when too much current flows through it, therebyinterrupting the current. An advantage of fuses is that they are passivecomponents that do not consume power. The quiescent current through afuse is zero. Thus, when the electrical circuit of the vehicle is inpark mode, the fuses consume no electrical energy. Another advantage isthat the fuses have the capacity of removing high power from a faultysystem. In other words, a fuse can drain high current and its quiescentcurrent is equal to zero whatever its draining capacity. However, a fusehas a main drawback because it is a sacrificial device. Once a fuse hasoperated, it must be replaced, which is not convenient in an automotiveenvironment.

It is known to use electronic switching devices, currently called“high-side switches” or “high-side drivers”, as an alternative to fuses.A high-side switch includes a transistor (such as a MOSFET transistor)for providing overcurrent protection and electronic means forcontrolling the transistor. In order to be able to switch high currents(typically currents between 100 mA and several hundreds of amps),high-side switches are provided with a MOSFET transistor of type N. Theycan safely drive high currents into complex grounded loads, incompliance with the automotive environment. However, high side switcheshave a high quiescent current, typically a few mA, compared to zero (fora conventional fuse).

Vehicles usually spend a lot of time in park mode, wherein current isdrained from the battery. In order not to drain too much energy from thebattery, the quiescent current of each device of the electrical circuitwithin the vehicle has to be very low. A quiescent current of somemilliamps is too high with respect to such a constraint of extremely lowelectrical consumption of the vehicle in park mode.

There is a need to improve the situation with an electrical safetysystem for providing overcurrent protection of an electrical circuit,for example in a vehicle, that can be reused after it has operated andhas also a very low quiescent current, for example in compliance theelectrical requirements in the automotive environment.

SUMMARY

A first aspect of the present disclosure concerns an electrical safetysystem for providing overcurrent protection of an electrical circuitincluding a main safety device having a first transistor for providingovercurrent protection. An auxiliary safety device includes a secondtransistor of P-type for providing overcurrent protection of theelectrical circuit. The main safety device and the auxiliary safetydevice provide the overcurrent protection alternately, respectively in afirst operating mode and in a second operating mode of the electricalcircuit. The auxiliary safety device includes a passive component,connected in series with the second transistor of P-type, for providinga voltage drop when a current passes through the passive component, anda driving circuit controlling turning off of the second transistor ofP-type under control of the voltage drop exceeding a first threshold.

Typically, the first operating mode consumes more electrical power thanthe second operating mode. Such an electrical safety system may beintegrated in a vehicle to protect an electrical circuit of the vehicle.

The present electrical safety system comprises a main safety deviceincluding a first transistor, for example a N-type transistor, and anauxiliary safety device including a P-type transistor. The twotransistors are alternately turned on to achieve in turn the function ofovercurrent protection, respectively in a first operating mode and in asecond operating mode of the electrical circuit. The auxiliary safetydevice comprises a passive component, connected in series with theP-type transistor, for providing a voltage drop when a current passesthrough said passive component, and a driving circuit controllingturning off of the P-type transistor under control of the voltage dropexceeding the first threshold, in the second operating mode.

Such a system allows to use a main safety device using a powertransistor (such as a N-type transistor) and capable of removing highcurrents from a faulty electrical circuit, in the first operating mode(for example when the vehicle is operated). In the second operating mode(for example when the vehicle is parked), this main safety device isturned off (deactivated) and does not consume any current. In order toachieve the overcurrent protection function in the second operatingmode, the auxiliary safety device provided with a P-type transistor isused. The overcurrent protection function is achieved by the P-typetransistor coupled with a passive component that provides a dropvoltage, when a current passes through the passive component, said dropvoltage being used as a command to control turning off the P-typetransistor.

The present configuration has the advantage of an extremely lowelectrical consumption (equal or very close to zero) due to the use ofthe passive component for providing the drop voltage and the P-typetransistor, in the second operating mode. In case of an excess ofcurrent flowing through the passive component, the latter provides ahigh voltage drop that controls turning off the P-type transistor andtherefore the current flow is interrupted. The overcurrent protectionfunction is thus achieved on the basis of structural elements (P-typetransistor and drop voltage passive component) that do not consumeelectrical energy. Therefore, the quiescent current of such a system iszero or very close to zero.

In the first operating mode, the main safety device is used and allowsto drain high currents in case of malfunction.

A controller, such as a controller of a vehicle integrating theelectrical safety system, may control turning on the first transistor ofthe main safety device and turning off the second transistor of theauxiliary safety device, in a first operating mode of the electricalcircuit, and control turning off the first transistor of the main safetydevice and turning on the second transistor of the auxiliary safetydevice, in a second operating mode of the electrical circuit.

A second aspect of the present disclosure concerns an electrical safetydevice for providing overcurrent protection of an electrical circuitincluding a transistor of P-type for providing overcurrent protection ofthe electrical circuit; a passive component, connected in series withthe transistor of P-type, for providing a voltage drop when a currentpasses through said passive component, and a driving circuit controllingturning off of the transistor of P-type under control of the voltagedrop exceeding a first threshold.

The passive component for providing the voltage drop may be a resistiveelement.

The driving circuit may include a first switch, controlled by thevoltage drop, that is turned off as long as the voltage drop does notexceed the first threshold and turned on under control of the voltagedrop exceeding the first threshold so as to output a control signal forcontrolling turning off the transistor of P-type.

The first switch may be either a bipolar transistor or a MOSFETtransistor. These types of transistor have a quiescent current that iszero.

The driving circuit may include a second switch through which the P-typetransistor of overcurrent protection is connected to the ground.

The second switch can be a transistor of N-type.

The driving circuit may include a latch component for controlling thesecond switch with an output signal depending on a control signalreceived through either a reset input or a set input, and the firstswitch outputs a control signal to the latch component.

The driving circuit may include a third switch, controlled by thevoltage drop, that is turned off as long as the voltage drop does notexceed a second threshold, said second threshold being less than thefirst threshold, and turned on under control of the voltage dropexceeding the second threshold and then transmit a wake-up signal.

The third switch may be either a bipolar transistor or a MOSFET.

In a particular embodiment, the first switch is a MOSFET and the thirdswitch is a bipolar transistor. The threshold voltage of a MOSFET isusually higher that the threshold voltage of a bipolar transistor. Sucha configuration allows to use the physical characteristics of these twodifferent types of transistors (MOSFET and bipolar) to define the first“high” threshold and the second “low” threshold.

A third aspect of the present disclosure concerns a vehicle integratingthe electrical safety system or the electrical safety device, aspreviously defined.

Other features, purposes and advantages of the disclosure will becomemore explicit by means of reading the detailed statement of thenon-restrictive embodiments made with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrical safety system for providing overcurrentprotection of an electrical circuit, according to a first embodiment;

FIG. 2 shows an auxiliary electrical safety device of the system of FIG.1 .

DETAILED DESCRIPTION

FIG. 1 shows an electrical safety system 100 for providing overcurrentprotection of an electrical circuit (not represented), according to afirst embodiment. For example, the system 100 may be integrated in avehicle, the electrical circuit having electrical and electronic devicesof the vehicle, supplied with electrical current by an automotivebattery of 12V for example. The electrical safety system 100 drainsexcess current resulting from a malfunction (such as an overload or ashort circuit) in the electrical circuit. More precisely, its mainfunction is to interrupt current flow in the electrical circuit whencurrent reaches an overcurrent detection value.

The electrical safety system 100 has a main safety device 1 and anauxiliary safety device 2. Both safety devices 1 and 2 are connected (inparallel) to an input terminal 3 a and an output terminal 3 b of theelectrical circuit. The main safety device 1 provides overcurrentprotection of the electrical circuit in a first operating mode of theelectrical circuit, while the auxiliary safety device 2 providesovercurrent protection of the electrical circuit in a second operatingmode of the electrical circuit. Thus, the main safety device 1 and theauxiliary safety device 2 provide the overcurrent protection of theelectrical circuit alternately, respectively in a first operating modeand in a second operating mode of the electrical circuit.

The first operating mode is expected to consume more electrical energythan the second operating mode. Typically, the first operating modecorresponds to a mode wherein the vehicle is operated, such as a drivemode, while the second operating mode corresponds to a mode wherein thevehicle is not operated (for example the vehicle is stopped and parked,the electrical circuit being in sleep mode). In the second operatingmode, the electrical circuit operates but with a very low electricalconsumption, in sleep mode. The main safety device 1 is intended to beused in the first operating mode, to provide the overcurrent protectionfunction, and deactivated in the second operating mode. On the otherhand, the auxiliary safety device 2 is intended to be used in the secondoperating mode, to provide the overcurrent protection function, anddeactivated in the first operating mode. In other words, the main safetydevice 1 and the auxiliary safety device are operated alternately toprovide the overcurrent protection function of the electrical circuit.

In the present embodiment, the main safety device 1 includes a firsttransistor 10, for example a N-type transistor, for providing theovercurrent protection function, connected between the input terminal 3a and the output terminal 3 b of the electrical circuit. The main safetydevice 1 further includes a charge pump 11 for driving transistor 10depending on a control signal “ON” (activation) or “OFF” (deactivation)from a controller 4 external to the main safety device 1. The chargepump 11 is connected to a command gate of the first transistor 10 and toa connection point 12 between input and output terminals 3 a and 3 b(for example between input terminal 3 a and transistor 10 as shown inFIG. 1 ). The control signal “ON” is for turning on (or activating, orswitching on) the first transistor 10 so that current can flow throughtransistor 10, while the control signal “OFF” is for turning off (ordeactivating or switching off) the first transistor 10, so that currentflow through transistor 10 is interrupted. The main safety device 1 isfor example a known high-side switch of the market. It has a goodcapacity to safely drive high currents into grounded loads, in case ofmalfunction or overload in the electrical circuit of the vehicle.

The auxiliary safety device 2 is an electronic device that provides anovercurrent protection function with an extremely low electricalconsumption, close to zero, by using the physical characteristics ofsome electrical components, as will be apparent later in thedescription. It has a draining capacity for removing current in case ofmalfunction that may be lower than the draining capacity of the mainauxiliary device 1.

A first embodiment of the auxiliary safety device 2 is now describedwith reference to FIG. 2 .

With reference to FIG. 2 , the auxiliary safety device 2 includes asecond transistor of P-type 20 for providing the function of overcurrentprotection of the electrical circuit of the vehicle, and a passivecomponent 21, connected in series with the P-type transistor 20, forproviding a voltage drop when current flows through said passivecomponent 21. For example, transistor 20 of overcurrent protection is aPMOSFET.

A driving circuit 22 is further provided to control turning off thesecond P-type transistor of overcurrent protection 20 under control ofthe voltage drop (provided by component 21) exceeding a first thresholdVrHi, in the second operating mode of the electrical circuit.

In the first embodiment, the driving circuit 22 generates a wake-up (oractivation) control signal under control of the voltage drop (providedby component 21) exceeding a second threshold V_(TH2), in the secondoperating mode of the electrical circuit.

The first and second thresholds V_(TH1) and V_(TH2) are voltage values.The second threshold V_(TH2) is less than the first threshold V_(TH1).

The basic function of the driving circuit 22 is to control switching offthe second transistor 20 of overcurrent protection in the secondoperating mode. The wake-up function of the driving circuit 22 isoptional.

The driving circuit 22 has a first switch 220 controlled by the voltagedrop 21 and which is turned off as long as the voltage drop provided bycomponent 21 does not exceed the first threshold V_(TH1) and is turnedon under control of the voltage drop 21 exceeding the first thresholdV_(TH1) so as to output a control signal for controlling turning off theP-type transistor 20.

In the first embodiment, the first switch 220 is a transistor. Forexample, the transistor 220 is either a bipolar transistor or a MOSFET(metal-oxide-semiconductor field-effect transistor). The transistor 220is connected to the two terminals of the voltage drop component 21, sothat the voltage drop controls operation of the transistor 220. If thetransistor 220 is a bipolar transistor, the base and emitter of thetransistor 220 are respectively connected to the two terminals ofvoltage drop component 21. If the transistor 220 is a MOSFET, the gridand source of the transistor 220 are respectively connected to the twoterminals of voltage drop component 21 so that the voltage dropcorresponds to VGS, that is the voltage between grid and source of thetransistor 220, and controls operation of the transistor 220.

The transistor 220 (first switch) operates as follows: it is blocked andoperates as an open switch, as long as the voltage drop is less than thefirst threshold V_(TH1), and it is saturated and operates as a closedswitch if the voltage drop exceeds the first threshold V_(TH1).

The first threshold V_(TH1) corresponds to the threshold voltage of thetransistor 220.

For example, the threshold voltage V_(TH1) is between 1 V and 5 V, forexample it is equal to 1,2 V.

The passive component 21, also called “voltage drop component”, has aresistive element. For example, passive component 21 has only aresistance. The resistive value of passive component 21 is adapted toprovide a voltage drop value equal to threshold voltage V_(TH1),suitable to control turning on a first switch 220, when a currentpassing through component 21 reaches or exceeds an overcurrent detectionvalue I_(TH1). The overcurrent detection value corresponds to an currentthreshold beyond which it is desired to achieve the overcurrentprotection function and interrupt the current flow in the electricalcircuit. In other words, the resistive value R of voltage drop component21 is determined to provide a voltage drop equal of V_(TH1) when thecurrent flowing through component 21 reaches the current thresholdI_(TH1) (or overcurrent detection value). More precisely, in the presentembodiment, the resistive value R of drop voltage component 21 is equalto V_(TH1)/I_(TH1). For example (the exemplary values only are given toillustrate the present disclosure):

if V_(TH1)=1,2 V and I_(TH1)=200 mA;

then R=V_(TH1)/I_(TH1)=1,2/0,2=6 Ω.

In the driving circuit 22, the switch 220 controls turning off (orswitching off) the P-type transistor 20 of overcurrent protectionthrough a second switch 221. The second switch 222 is for exampletransistor of N-type that controls the P-type transistor of overcurrentprotection 20. It has the function of connecting a command gate ofP-type transistor 20 to the ground. In other words, the command gate ofP-type transistor 20 is connected to the ground through the N-typetransistor 222.

The second switch 221 is controlled through an electronic latchcomponent 223. Latch component 223 includes a set input “S”, a resetinput “R” and an output having two stable states that are set accordingto control signals received through set or reset inputs. Advantageously,latch component 223 includes only MOSFET and/or bipolar transistors toachieve the latch function while avoiding any quiescent current. Theoutput of latch component 223 is connected to a command gate of switch221. The latch component 223 outputs a control signal for controllingthe second switch 221. The set input and the reset input of latchcomponent 223 are connected to the external controller 4, here throughtwo respective links. The set input receives a control signal “ON”(activation), from the external controller 4, for turning on (orswitching on) the second switch 221 and consequently turning on (orswitching on) the P-type transistor 20. The reset input receives acontrol signal “OFF” (deactivation), from the external controller 4, forturning off (or switching off) the second switch 221 and consequentlyturning off (or switching off) the P-type transistor 20.

The first switch 220 is also connected to the reset input of latchcomponent 223 and transmits a control signal “OFF” (deactivation) to thereset input of latch component 223, in order to turn off second switch221 and consequently P-type transistor 20, upon activation (or switchingon), as will be explained later.

In the first embodiment, the driving circuit 22 further has a thirdswitch 224 for generating a wake-up or activation signal when thecurrent flow through voltage drop component 21 increases a little, dueto a normal action of one or more devices of the vehicle, in the secondoperating mode. The function of this switch 224 is to transmit a wake-upsignal to the electrical circuit, in the second operating mode, when thecurrent flowing through the drop voltage component 21 exceeds the secondthreshold value V_(TH2), that is less than the first threshold valueV_(TH1). This wake-up signal can be used to reactivate the main safetydevice 1, either directly or through controller 4, because it isexpected that the electrical circuit will soon go into the firstoperating mode. The third switch 224 is controlled by the voltage drop,as first switch 220, and is: turned off as long as the voltage drop(provided by component 21) does not exceed the second threshold V_(TH2),and turned on under control of the voltage drop exceeding the secondthreshold V_(TH2) and then transmit a wake-up signal to the electricalcircuit.

In the first embodiment, the third switch 224 is a transistor. Forexample, the transistor 224 is either a bipolar transistor or a MOSFET(metal-oxide-semiconductor field-effect transistor). The transistor 224is connected to the two terminals of the voltage drop component 21, sothat the voltage drop controls operation of the transistor 224. If thetransistor 224 is a bipolar transistor, the base and emitter of thetransistor 224 are respectively connected to the two terminals ofvoltage drop component 21. If the transistor 224 is a MOSFET, the gridand source of the transistor 224 are respectively connected to the twoterminals of voltage drop component 21 so that the voltage dropcorresponds to V_(GS), that is the voltage between grid and source ofthe transistor 224, and controls operation of the transistor 224. Thetransistor 224 operates as follows: it is blocked and operates as anopen switch, as long as the voltage drop is less than the secondthreshold V_(TH2), and it is saturated and operates as a closed switchif the voltage drop exceeds the second threshold V_(TH2).

The second threshold V_(TH2) corresponds to the threshold voltage oftransistor 224. Advantageously, the threshold voltage V_(TH2) is between0,6 V and 1 V. For example, the threshold voltage V_(TH2) is equal to0,7 V. In the illustrative example previously given, the resistive valueR of voltage drop component 21 is equal to 6 ω. In that case, the switch224 is turned on if the current that flows through component 21 exceedsV_(TH2)/R=0,7/6≈116 mA.

Since the second threshold value V_(TH2) is less than the firstthreshold value V_(TH1), the voltage drop may exceed V_(TH2) withoutreaching or before reaching V_(TH1). In such a case, the third switch224 allows to wake up one or more devices of the electrical circuit inthe vehicle, in particular the main safety device 1. When the mainsafety device 1 is reactivated, the calculator 4 (controller) of thevehicle controls turning off the P-type transistor 20 of overcurrentprotection of the auxiliary safety device 2, by transmitting a controlsignal OFF to the reset or “R” input of latch component 223.

The controller 4 is provided to control the operation of the main safetydevice 1 and the auxiliary safety device 2. More precisely, thecontroller 4 controls: turning on the N-type transistor of overcurrentprotection 10 of the main safety device 1 and turning off the P-typetransistor of overcurrent protection 20 of the auxiliary safety device2, in the first operating mode of the electrical circuit; turning offthe N-type transistor of overcurrent protection 10 of the main safetydevice 1 and turning on the P-type transistor of overcurrent protection20 of the auxiliary safety device 2, in the second operating mode of theelectrical circuit, the first operating mode consuming more electricalpower than the second operating mode.

The controller 4 is, for example, a calculator of the vehicle.

The operation of the safety system 100 will now be described.

When the vehicle is operated for example in drive mode, its electricalcircuit is in the first operating mode. In this first operating mode,the main safety device 1 is operated to provide the overcurrentprotection function of the electrical circuit, in a conventional manner.The N-type transistor 10 of overcurrent protection is turned on, undercommand of the controller 4, and operates as a closed switch, so thatcurrent can flow through the transistor 10 and input and outputterminals 3 a, 3 b of the electrical circuit of the vehicle. The inputand output terminals 3 a, 3 b are connected to one another through theN-type transistor 10 (closed switch).

In this first operating mode, the auxiliary safety device 1 isdeactivated. It means that the P-type transistor of overcurrentprotection 20 is turned off and operates as an open switch.Consequently, no current flows through the voltage drop component 21.The drop voltage is zero and therefore the first and third switches 220,224 are also turned off and operate as open switches. Deactivation ofthe auxiliary safety device 1 is usually triggered by an impulse signal“OFF” transmitted from the controller 4 to the set input of latchcomponent 223. As a result, the output signal from latch component 223controls the second switch 221 to be turned off and to operate as anopen switch. In this state, the transistor 20 is not connected to theground and therefore is turned off.

Then, we assume that the vehicle is stopped and parked. Therefore, itselectrical circuit goes into the second operating mode (or “park mode”or “sleep mode”). In this second operating mode, the electronic devicesof the vehicle go into sleep mode and their electrical consumption iszero or close to zero. To enter the second operating mode, thecontroller 4 transmits a control signal OFF for deactivating the mainsafety device 1 and, at the same time, a control signal ON to theauxiliary safety device 2 to activate the auxiliary safety device 2.

Upon reception of the control signal OFF from controller 4 by the mainsafety device 1, the charge pump 21 is stopped and consequently N-typetransistor 10 of overcurrent protection is turned off and operates as anopen switch.

In the auxiliary safety device 20, the control signal ON from controller4 is received by the set input of latch component 223. Consequently, theoutput signal from latch component 223 is changed and therefore controlsturning on the second switch 221. Thus, the second switch 221 operatesas a closed switch under control of the output signal from latchcomponent 223. As a result, the command gate of the P-type transistor 20is connected to the ground through the second switch 221 which resultsin turning on the P-type transistor 20 that operates as a closed switch.The input and output terminals 3 a, 3 b of the electrical circuit arethus connected to one another through the voltage drop component 21 andthe P-type transistor 20.

In the second operating mode, the current through the voltage dropcomponent 21 is usually equal or close to zero, since most of theelectronic devices of the electrical circuit are in sleep or standbymode. As a result, the voltage drop provided by component 21 is equal orclose to zero. So, the first and third switches are turned off andoperate as open switches.

In the second operating mode, according to a first scenario, the currentflowing through voltage drop component 21 increases a little, due to anormal action achieved by one or more electronic devices of the vehicle.Such a small increase of current results in a small increase of thevoltage drop provided by component 21. Such a scenario may occur forexample when a user with a contactless key comes close to the vehicle.The contactless key wakes up some devices within the vehicle, whichresults in an increase of the current in the electrical circuit. If theresulting voltage drop exceeds the second threshold voltage V_(TH2), ithas the effect of turning on the third switch 224. The third switch 224switches on, under control of the voltage drop exceeding V_(TH2), andtherefore transmits a wake-up signal to electrical circuit. For example,the wake-up signal from the third switch 225 is used to activate or wakeup the controller 4 and then the controller 4 can activate the mainsafety device 1 by transmitting a signal ON to the charge pump 21 anddeactivate the auxiliary safety device 2 by transmitting a signal OFF tothe latch component 223. As previously indicated, the wake-up functionprovided by the third switch 224 is optional.

As long as the voltage drop remains below the first threshold valueV_(TH1), the first switch 220 is maintained turned off and continues tooperate as an open switch.

In the second operating mode, according to a second scenario, thecurrent flowing through voltage drop component 21 increases too much,which results in an important increase of the voltage drop, due forexample to a malfunction (overload, short circuit, etc.) in theelectrical circuit. In such a case, the voltage drop exceeds not onlythe first threshold voltage V_(TH1) but also the second thresholdvoltage V_(TH2), usually in a very short period of time. The firstswitch 220 is turned on under control of the voltage drop exceeding thefirst threshold V_(TH1). As a result, the first switch 220 outputs acontrol signal that is transmitted to the reset input of latch component223, which, in turn, controls turning off the second switch 221 undercontrol of the output signal from latch component 223 (which state ischanged). Upon switching off the second switch 221, the P-typetransistor 20 of overcurrent protection is no longer connected to theground and is consequently turned off. As the transistor 20 is turnedoff, the current flow through the transistor 20 is interrupted. Theovercurrent protection is thus achieved.

In the above second scenario of the voltage drop exceeding the firstthreshold V_(TH1), the voltage drop also exceeds the second thresholdV_(TH2) and, consequently, the third switch 224 is turned on andtransmits a wake-up signal to the electrical circuit.

In a particular embodiment, the first switch 220 is a MOSFET and thethird switch 224 is a bipolar transistor. Such a structure isadvantageous because the threshold voltage of a bipolar transistor isusually less than the threshold voltage of a MOSFET transistor. Thus, itallows to provide a “high” voltage threshold V_(TH1), for theovercurrent protection function, and a “low” voltage threshold V_(TH2),by simply using the physical characteristics of these two differenttypes of transistor.

In the first embodiment, the resistive value of drop voltage component21 is determined according to a fixed overcurrent detection value (forexample 200 mA) and the threshold voltage of transistor 220 of the firstswitch (for example 1,2V).

In a variant, the threshold voltage V_(TH2) is not equal to thethreshold voltage of the transistor 224 but depends on the thresholdvoltage of the transistor 224 coupled with a resistive element.

In another variant, the threshold voltage V_(TH1) is not equal to thethreshold voltage of the transistor 220 but depends on the thresholdvoltage of the transistor 220 coupled with a resistive element.

Such configurations allow to adjust the threshold voltage values V_(TH1)and/or V_(TH2).

A second embodiment of the electrical safety system is analogous to thefirst embodiment and only differs in that the auxiliary safety device adoes not include a third switch 224.

The preceding description is illustrative rather than limiting innature. Variations and modifications of the disclosed exampleembodiments may become apparent to those skilled in the art that do notnecessarily depart from the essence of the invention. The scope of legalprotection provided to the invention can only be determined from thefollowing claims.

We claim:
 1. An electrical safety system for providing overcurrentprotection of an electrical circuit, the electrical safety systemcomprising: a main safety device including a first switch forselectively providing a first level of overcurrent protection of theelectrical circuit during a first operating mode in which the electricalcircuit includes a first amount of electrical energy consumption; anauxiliary safety device including a second switch for selectivelyproviding a second level of overcurrent protection of the electricalcircuit during a second operating mode in which the electrical circuitincludes a second amount of electrical energy consumption, and a voltagedrop component in electrical series with the second switch for providinga voltage drop when current passes through the voltage drop componentduring the second operating mode, wherein the second level ofovercurrent protection is lower than the first level of overcurrentprotection and the second amount of electrical energy consumption islower than the first amount of electrical energy consumption; and adriving circuit for controlling the second switch to protect theelectrical circuit from overcurrent when the voltage drop exceeds apredetermined threshold.
 2. The system according to claim 1, wherein theelectrical circuit is installed on a vehicle including electronicdevices, the first operating mode corresponds to the electronic devicesbeing turned on, and the second operating mode corresponds to theelectronic devices of the vehicle being turned off or in a sleep mode.3. The system according to claim 2, comprising a controller that isconfigured to activate the main safety device and deactivate theauxiliary safety device when the electronic devices are turned on, andactivate the auxiliary safety device and deactivate the main safetydevice when the electronic devices are turned off or in the sleep mode.4. The system according to claim 1, wherein the voltage drop componentcomprises a passive, resistive element.
 5. The system according to claim1, wherein the first switch is either a bipolar transistor or a MOSFETtransistor, and the second switch comprises a P-type transistor switch.6. The system according to claim 5, wherein the driving circuitselectively connects the P-type transistor switch to ground when thevoltage drop exceeds the predetermined threshold.
 7. The systemaccording to claim 6, wherein the driving circuit comprises a secondswitch that is a transistor of N-type, and the second switch selectivelyconnects the P-type transistor switch to ground.
 8. The system accordingto claim 4, wherein the driving circuit comprises a latch component forcontrolling the second switch with an output signal depending on acontrol signal received through either a reset input or a set input ofthe latch component.
 9. The system according to claim 7, wherein thedriving circuit includes a third switch, controlled by the voltage drop;the third switch is turned off as long as the voltage drop does notexceed a second threshold that is less than the predetermined threshold;the third switch is turned on under control of the voltage dropexceeding the second threshold; and the driving circuit transmits awake-up signal when the third switch is turned on.
 10. The systemaccording to claim 9, wherein the third switch is either a bipolartransistor or a MOSFET transistor.
 11. The system according to claim 9,wherein the first switch comprises a MOSFET transistor and the thirdswitch is a bipolar transistor.
 12. The system according to claim 1,wherein the first switch comprises an N-type transistor.
 13. A vehiclecomprising the electrical safety system according to claim
 1. 14. Amethod of providing overcurrent protection for an electrical circuit,the method comprising: providing a first level of overcurrent protectionfor the electrical circuit through a main safety device during a firstoperating mode in which the electrical circuit includes a first amountof electrical energy consumption; and providing a second level ofovercurrent protection for the electrical circuit through an auxiliarysafety device during a second operating mode in which the electricalcircuit includes a second amount of electrical energy consumption, bycontrolling the auxiliary device based on a voltage drop exceeding apredetermined threshold, wherein the second level of overcurrentprotection is lower than the first level of overcurrent protection andthe second amount of electrical energy consumption is lower than thefirst amount of electrical energy consumption.
 15. The method accordingto claim 14, wherein the electrical circuit is installed on a vehicleincluding electronic devices, the first operating mode corresponds tothe electronic devices being turned on, and the second operating modecorresponds to the electronic devices of the vehicle being turned off orin a sleep mode.
 16. The method according to claim 15, comprising:activating the main safety device and deactivating the auxiliary safetydevice when the electronic devices are turned on, and activating theauxiliary safety device and deactivating the main safety device when theelectronic devices are turned off or in the sleep mode.